The present invention relates to an acoustic attenuation device, comprising two substantially parallel plates defining a rectangularly shaped space, noise detection means arranged between the two plates, inverse noise emission means arranged between the two plates, and control means for controlling the inverse noise emission means in such a way as to minimize a quantity supplied by the noise detection means.
Applications of the invention are, for example, in the field of sound insulation of premises, in particular with double glazing, in the production of cowlings for equipment that generates noise, or in the field of insulating the passenger compartments of means of transport. An important application is in the field of double glazings.
A device of the type indicated above, termed active double wall, relies on the operating principle summarized below.
The mass-spring-mass resonant frequency of a double wall constituted by two parallel rectangular plates separated by an air sheet of thickness d is given by the equation: ##EQU1## with: .rho..sub.0 : density of the medium locate between the plates (1.18 kg/m.sup.3 in the case of air)
c.sub.0 : speed of sound in the medium located between the plates (340 m/s in the case of air). ##EQU2## m.sub.1, m.sub.2 : mass per unit area of the plates (in kg/m.sup.2) PA0 .phi..sub.lmn : modal base associated with the cavity in question. PA0 .omega.: angular frequency (=2.pi.f) PA0 x,y: spatial coordinates parallel to the plates PA0 z: spatial coordinate perpendicular to the plates PA0 t: time.
This resonant frequency generally lies between 50 and 250 Hz.
Overall, for a given frequency f, the acoustic behavior of a double wall is considered to be as follows:
f&lt;f.sub.mrm : the two plates vibrate in phase. The variation in volume between the plates remains small. The double wall behaves as a single wall of equivalent mass. PA1 f.apprxeq.f.sub.mrm : the two plates, strongly coupled by the air sheet, vibrate in phase opposition. This leads to large variations in volume of the air sheet (phenomenon of "breathing" of the plates) and to poor acoustic insulation by the double wall. PA1 f&gt;f.sub.mrm : the movements of the two plates are decoupled by the air sheet. The acoustic insulation of the wall then increases rapidly with frequency.
The attenuation device aims to compensate for the poor acoustic insulation provided by the double wall close to f.sub.mrm. The principle consists in preventing, by means of an electro-acoustic system, any variation in volume of the air sheet.
The acoustic pressure field in the air sheet can be written in the form of a modal series: ##EQU3## with: .alpha..sub.lmn : amplitude of mode l,m,n
In the case of a parallelepipedally shaped air sheet: EQU .phi..sub.lmn (x,y,z)=cos(l.pi.x/L.sub.x)cos(m.pi.y/L.sub.y)cos(n.pi.z/L.sub.z)(3)
L.sub.x, L.sub.y, L.sub.z (=d): dimensions of the air sheet
The eigenfrequency f.sub.lmn of a mode with indices (l,m,n) of the air sheet is given by the equation: ##EQU4##
The variation in volume of the air sheet is directly proportional to the amplitude of the (0,0,0) mode, without the amplitude of the other modes close to the resonant frequency f.sub.mrm of the wall being affected. However, it is difficult to measure and excite only this mode by actions which, a priori, involve all the modes. Indeed, the expression given above (2) for the acoustic pressure shows that the measurement taken by a microphone will include the responses of modes other than the (0,0,0) mode.
It is desirable, in order to obtain efficient attenuation, to reduce the contribution, in the quantity to be minimized, of the low-frequency modes other than the (0,0,0) mode, and to operate so that the inverse noise emission means excite the (0,0,0) mode predominantly while exciting the other modes of the air sheet as little as possible.
One object of the invention is thus to improve the efficiency of the attenuation provided by an active double wall device.